Several golden age characters first appeared in the context of the silver age world of Earth-One, wherein it was thought that Superboy was that world's first hero approximately twenty-some years prior to the presently published tales. For instance, the Guardian appeared with his Newsboy Legion aiding that world's Jimmy Olsen, Manhunter teamed up on one occasion with the modern Batman, and Air Wave was identified as the cousin of Hal Jordan, the Green Lantern. Both were Jordans, after all, so why not tie them together?
Then the decidedly different All-Star Squadron was published, and all these characters were members of that team back in the 1940's when their tales were published. Further, the series Who's Who which was published concurrent with the Crisis on Infinite Earths listed each character's history (as well as that of Zatara, Sargon, and Commander Steel) from their joining the All-Stars to their modern-day meetings of Earth-One's heroes. This was then explained in an All-Star letter column as individuals who migrated from Earths Two to One (and this was further solidified during a team-up of the Justice League and Justice Society by a comment Flash made to teammate Power Girl when she met Sargon)
Yet how could Air Wave be cousins with Mr. Jordan? Further, how could Jim Harper, the Guardian, be the uncle of Roy Harper aka Speedy? We can explain this quite simply. When he moved to Earth-One, Larry Jordan sought out familial connections. Instead, he met Helen Jordan and fell in love with her. She was the actual cousin of Hal, Larry was but an in-law. And the Guardian? Remember there were TWO Roy Harpers who would become Green Arrow's sidekick Speedy. While uncle to the Earth-Two version, Jim Harper himself sought out family when moving to Earth-One and was the "long lost" uncle of the orphaned lad. This is not unheard of, for Earth-Two's Huntress referred to her father's Earth-One counterpart Bruce Wayne as "Uncle Bruce".
Their presence on this new world, along with that of others, could upset the cosmic balance? Ah, that is where the Crisis comes in and matters are balanced once more. More on that in Part 6 of our comprehensive Migrants of Multiple Earths Thesis.